Intervertebral discs, which lie between adjacent vertebrae in the spine, provide the vertebral column with the necessary biomechanical properties for movement and compressive strength. Each disc forms a cartilaginous joint to allow slight movement of the vertebrae and acts as a ligament to hold the vertebrae together. Intervertebral discs are composed of three tissue types, namely, the annulus fibrosis (AF), the nucleus pulposis (NP) and the end plates. The AF consists of a series of loosely connected concentric layers of collagen tissue that surround the NP and distribute pressure evenly across the disc. The NP is formed from a network of collagen fibrils that are loosely embedded in a gelatinous matrix of proteoglycan aggrecans and chondrocytes. Alteration of the biomechanical properties of intervertebral discs is suspected to be the leading cause of degenerative disc disease and of debilitating back pain. A diverse range of factors, including mechanical, biochemical, nutritional, and genetic factors, are known to contribute to the development of degenerative disc disease. Examples include: abnormal mechanical loading, genetic predisposition, and reduced cellular activity.
The treatment of pain associated with degenerative disc disease usually involves conservative treatments, such as physical therapy and medication. Surgical intervention, usually by discectomy, is reserved for those patients who fail conservative treatment. Discectomy provides relief from pain originating from the compression of neural structures by the surgical removal of disc tissue; however, such surgeries do not treat the underlying biological problem. Spinal fusion, a surgical technique used to join two or more vertebrae, is another surgical intervention option that is often performed in combination with discectomy to immobilize the affected vertebrae. Supplementary bone tissue (e.g., autologous iliac crest bone, allograft tissue, synthetic cage with bone substitute filler) is used in conjunction with the body's natural bone growth processes to fuse the vertebrae. However, autologous tissue recovery often leads to graft site morbidity. Furthermore, fusion often leads to adjacent segment disease, which may limit the duration of success of the operation. Thus, there remains a need for safe, effective methods for treating degenerative disc disease.